Graphite based materials and particularly artificial graphite powder are primarily used as carbon materials constituting negative electrodes in lithium ion secondary batteries.
There has been much research aimed at increasing the capacity per unit mass of graphite based materials in order to increase the capacity of lithium ion secondary batteries. As a result of such efforts, a graphite based material exhibiting a capacity exceeding 360 mAh/g (compared to the theoretical capacity of graphite of 372 mAh/g) has been developed, and increases in the capacity of graphite based materials themselves have nearly reached a limit.
Under such conditions, it has been attempted to increase the capacity of batteries by further compressing a negative electrode material and increasing the packing density of electrodes. In order to compress and compact a graphite based material, it is necessary to deform graphite particles and fill vacant spaces. Soft graphite particles are suitable for this purpose.
A highly crystalline graphite based material such as natural graphite easily is extremely soft and easily deforms since it readily undergoes interlayer sliding. Therefore, it is easy to increase electrode density with this material. However, when subjected to a slight pressure, closed pores develop in an electrode, and the charge acceptance of the electrode ends up decreasing.
A material comprising a soft, highly crystalline graphite such as natural graphite having its surface covered with low crystallinity carbon or having such carbon locally adhered to its surface undergoes suppressed interlayer sliding in the interior graphite due to the low crystallinity carbon which has a high hardness. Therefore, closed pores do not readily develop, and a decrease in charge acceptance is suppressed. However, the high hardness of particles causes an increase in the applied pressure required to compress a negative electrode material, and the electrode density cannot be increased in cases when the negative electrode material cannot be adequately compressed due to apparatus limitations. If excess compression is carried out in order to obtain a high electrode density, marked collapse of the surface coating of the negative electrode material occurs, leading to a large decrease in high temperature storage properties (a decrease in capacity when a battery having an electrode using the negative electrode material is stored at a high temperature). In addition, hard particles do not experience sufficient contact between particles. Therefore, storage properties worsen due to the contact resistance between particles which increases after high temperature storage.
In this respect, Patent Document 1 proposes mixing flake graphite particles with graphite particles having their surface coated with amorphous graphite. The flake graphite particles are easily crushed and can act as a cushion. Therefore, according to that document, as a result of this mixing, collapse of the coated graphite particles when an electrode is subjected to rolling can be suppressed, and an increase in the specific surface area can be suppressed, thereby suppressing a decrease in the charging and discharging efficiency.
Patent Document 2 describes mixing of natural flake graphite which has undergone spheroidizing pulverization and then heat treatment at a high temperature with graphite particles having their surface coated with amorphous carbon or the like (see claim 4 and Example 6, for example).
Patent documents 3 and 4 propose mixing graphite which is coated with non-graphitic carbon (referred to below as coated graphite) with uncoated graphite.